Method for producing a screen printing stencil with a laser

ABSTRACT

In the production of a screen printing stencil, a thin-walled hollow cylinder having a light responsive layer on the outside is rotated about its cylinder axis and exposed by means of a laser beam impinging on it. The laser beam is focused in the region of the light responsive layer. The laser beam is moved in the direction of the cylinder axis, and is switched on and off in agreement with a desired stencil pattern. A radial deviation of the actual position of the wall of the hollow cylinder from its ideal position is determined from at least one measuring position fixed relative to the laser beam for a multiplicity of circumferential positions of the hollow cylinder. A first actuating signal is then derived from at least one of the radial positional deviations obtained at the measuring positions. The stencil pattern is displaced in accordance with the first actuating signal in the circumferential direction of the hollow cylinder for the purpose of compensating a tangential deviation of the hollow cylinder wall from its ideal position.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and device for accurately producing ascreen printing stencil by compensating for deviations.

2. Description of Related Art

Present methods for producing a screen producing stencil includesexposing a thin-walled hollow cylinder bearing a lacquer layer on theoutside, while rotating the cylinder about its cylinder axis, by meansof a laser beam impinging on it, the focus of the laser beam coming tolie in the region of the lacquer layer. The laser beam itself is movedin the direction of the cylinder axis and is also switched on and off inagreement with a desired stencil pattern, in order to transfer thisstencil pattern into the lacquer layer.

The device used for this purpose has a bearing device which serves thepurpose of rotatably accommodating the thin-walled hollow cylinder.Furthermore, a carriage is present which can be displaced along thecylinder axis of the hollow cylinder and carries a deflecting opticalsystem for deflecting the laser beam onto the lacquer layer. A switchingdevice is used to undertake the switching on and off of the laser beamin agreement with the stencil pattern stored in a storage device.

SUMMARY OF THE INVENTION

It is the object of the invention to produce the stencil pattern on thescreen printing stencil even more accurately.

This and other objects of the invention are fulfilled by providing amethod of fixing at least one measuring position relative to the laserbeam; determining a radial deviation of the actual position of the wallof the hollow cylinder from its ideal position for a multiplicity ofcircumferential positions of the hollow cylinder; and deriving a firstactuating signal from at least one of the radial positional deviationsobtained at the measuring position, in order to compensate for atangential deviation of the hollow cylinder wall from its ideal positionto displace the stencil pattern in the circumferential direction of thehollow cylinder.

Form variations in the hollow cylinder which are caused by the staticand/or dynamic influences and which can lead to an undesireddisplacement of the cylinder surface relative to the laser beam can becompensated in this way, with the result that the stencil patterngenerated on the hollow cylinder agrees even more accurately with thedesired stencil patterns, which may be stored in an electronic memory.

According to an advantageous embodiment of the method of the presentinvention, the method may further include deriving a second actuatingsignal at least from the radial positional deviation at the measuringposition, in order to adjust the focus of the laser beam in the radialdirection after the hollow cylinder has rotated over a circumferentialsection corresponding to the angular distance between the measuringposition and the laser beam, in order to compensate this radialpositional deviation.

The focus of the laser beam can thus always be held in the region of thelight responsive layer by means of this measure, even if the cylindersurface were to be moved in the radial direction by static and/ordynamic influences, as a result of which a more accurate stencil patternis likewise obtained.

The objects of the present invention are also fulfilled by providing adevice including an optics carriage on which at least one sensor formeasuring a radial deviation of the actual position of the wall of thehollow cylinder from its ideal position is mounted, as well as adjustingmeans which carry out a tangential displacement of the stencil patternas a function of the measured positional deviation. This tangentialdisplacement can be performed by electrical adjusting means which ensurean earlier or later retrieval of the stencil pattern from the storagedevice, in order in this way to compensate the form variation of thehollow cylinder in the tangential direction.

However, it is also possible to use as adjusting means electromechanicaladjusting means which appropriately deflect the laser beam impinging onthe hollow cylinder in the circumferential direction of the hollowcylinder. It is also possible to use further adjusting means to carryout an adjustment of the focus of the laser beam in the radial directionrelative to the hollow cylinder as a function of the measured radialpositional deviation, in order to compensate for radial form variationsof the hollow cylinder from its ideal, circular form.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

FIG. 1 illustrates form errors in the case of a real screen printingstencil;

FIG. 2 illustrates changes in the case of a circular cylindrical screenprinting stencil mounted eccentrically relative to an axis of rotation;

FIG. 3 illustrates membrane vibrations and amplitudes of vibration inthe case of a circular cylindrical screen printing stencil;

FIG. 4 illustrates sublengths of the screen printing stencil;

FIG. 5 shows a perspective total view of a device according to thepresent invention;

FIG. 6 shows a cross-section in the region of an optics carriage of thedevice according to FIG. 5;

FIG. 7 shows a longitudinal section through a screen printing stencil;

FIG. 8 shows a side view of an optics carriage with electromechanicaladjusting means;

FIG. 9 shows a plan view of the optics carriage according to FIG. 8;

FIG. 10 shows a side view of a further optics carriage withelectromechanical adjusting means;

FIG. 11 shows a plan view of the optics carriage according to FIG. 10;and

FIG. 12 shows a circuit arrangement which can be understood as anelectrical adjusting device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

When engraving circular cylindrical screen printing stencils on amachine specifically for this purpose, i.e., an engraving machine, anaccurate concentricity is-very important for the error-free matching ofthe different colors which are printed by means of a set of suchstencils. Deviations of the cross-sections of the stencil from thecircular Shape and eccentric positions of the center of across-sectional circle with respect to the axis of rotation of theengraving machine cause pattern images on the screen cylinder which areinaccurately engraved and displaced in the circumferential direction.

Giving consideration to a real round stencil, that is to say a verythin-walled perforated circular hollow cylinder, for example, whichnormally has a diameter of 200 to 300 mm, a length of 1000 to 3000 mmand a wall thickness of 0.1 mm, it may be stated that with respect tothe targeted circular cylindrical shape, the real round stencil has formvariations which can amount to a few tenths of a millimeter. Of course,when determining these form variations, it is important how the hollowcylinder is clamped during the measurement. If, for example, the roundstencil is placed with one of its end cross-sections on the ground, theform variations of the real round stencil can even increase up to a fewmillimeters with respect to an ideal circular cylinder of the same size.It was assumed in the case of the above information on the formvariations that the two end faces of the stencil were retained byaccurately concentric clamping devices, e.g., the faces were clamped onaccurately concentric internal chucks, or that concentric conical endpieces of the type of the work-holding centers of a lathe engage in theopen end faces of the stencil.

Given appropriately accurate design of these clamping devices, the thinround stencil will exhibit a sufficiently slight eccentricity at itsends, but will show ever stronger deviations from concentricity towardsits center because of internal stresses. If these deviations aremeasured by means of a contactless measuring method, for exampleinductively or optically the radial eccentricities of a few tenths of amillimeter already mentioned above are found towards the center of thestencil. The measurement of the radial eccentricities is finallyintended to permit compensation of these errors by suitable technicalmeasures, with the result that engraving errors are avoided. Themeasuring device is denoted here as a sensor, and is intended to senseoptically or inductively, but any contactless method may be used toestablish the perpendicular distance between a reference pointpermanently connected to it and the stencil wall moving past.

Physical variables of the environment of the stencil influence theradial eccentricities, as a result of which these deviations frominternal stresses are either amplified or diminished. Thus, for example,a pressure which is applied in any way at all to the inside of thestencil acts on the latter to round it out, i.e., the stencil is moreclosely approximated to the shape of a circular cylinder by the membranestresses which form.

Temporarily variable forces acting on the stencil excite the latter tovibrations, and can amplify the deviations. Such forces are released,for example, by poor rotary actuators. As experiments have shown, thesedynamic deviations of the hollow cylinder wall from concentricity, whichare superimposed on the static form errors, can likewise be diminishedby a pressure acting on the inside of the stencil. The pressureevidently dampens the vibrations of the stencil, something which will bediscussed further below. However, it is not mandatory.

The geometrical description of the errors requires suitable coordinatesto be established with respect to the stencil and the measuring device.Each stencil bears somewhere on its circumference a zero mark or apasser mark whose position is initially fixed arbitrarily, but remainspermanently connected to the stencil. The polar coordinate systemlikewise permanently connected to the stencil can be defined by means ofthis mark in the following way, as is explained with the aid of FIG. 1:

The origin of the coordinate system lies on the axis of rotation DA ofthe stencil.

The radial direction is normal to this axis.

The angle χ=0 is given by the connection with the zero mark indicated atNM.

The orientation of the angular coordinate χ is such that values for χare to be counted positive when they point against the direction ofrotation of the stencil.

The angular position φ of the rotating stencil with respect to thesensor is measured between the latter and the zero mark provided on thestencil. It holds under these preconditions that

    φ=χ.                                               (1)

In the case of all engraving machines, provision is made for the purposeof measuring the angle φ of rotation of an encoder which transmits azero pass agreeing with the position of the zero mark, and, in addition,subdivides the angle of a full rotation (360 degrees=2π) into N_(um)pulses. The N_(um) -th pulse of the preceding rotation is transmitted incongruence with the zero pulse of the following rotation. In order toconvert from the angle φ of rotation to the k-th pulse, and vice versa,relations ##EQU1## can be formed. Since k can only be an integer, it isnecessary in the case of this angle measuring technique to be limited todiscrete, but certainly very fine, angular steps for φ.

The description of the error picture is best done by expanding thedeviations, occurring over one rotation of the stencil, in the radius ofthe real stencil with respect to an ideal stencil, that is to say acircular cylindrical stencil, mounted concentrically with respect to theaxis of rotation, having the constant radius R₀ in a Fourier series.##EQU2##

This is the representation of the stencil radius in the definedstencil-fixed coordinate system. The distance measured from the sensorto the rotating real stencil is then s(χ)=A-R(χ) and with the use of theangular position φ between the zero mark and the sensor it holds becauseof equation (1) that ##EQU3##

In this representation, the series term of the zeroth order s₀ =A-R₀ isthe distance of the sensor from the ideal stencil. The term of firstorder c_(i) corresponds to a simple eccentricity of the center of thecross-sectional circle of the stencil which has just been measured. Theterm c_(i) can also be of considerable magnitude in the case of anaccurately circular cylindrical cross-section. When looking at a fewcross-sections of the stencil which follow one another in thelongitudinal direction, the terms of first order respectively applyingto these cross-sections correspond to a deviation in the longitudinalaxis of the stencil from a straight line, and thus represent a bendingof this longitudinal axis. It is to be noted that this bending cannot bediminished by a pressure on the inside of the stencil.

The series term of second order corresponds to a deviation in thecross-section resulting in an oval, and can be led towards zero even byslight pressure actions on the inside of the stencil. The series term ofthird order corresponds to a deviation of the stencil cross-section fromconcentricity resulting in a triangle, and can likewise be compensatedby an internal pressure. A similar statement holds for all the followingdeviations of higher order. However, the compensation of the radialeccentricity requires a higher internal pressure the higher the Fourierorder of deviation. It can be shown that the radial eccentricities canbe returned to zero completely only by an infinitely high internalpressure. In the case of a finite internal pressure, a residual errorremains which is proportional to the amplitude c_(i) of the error seriesterm, which can be established in the case of a stencil without internalpressure.

Measuring and processing the radial eccentricity in the case of astencil stabilized and rounded out by internal pressure:

It is now presupposed that a round stencil is present which is largelyrounded out by a pressure acting on its interior, and which also doesnot vibrate because of this internal pressure. The stabilization bymeans of internal pressure can be explained as follows, for example.Vibrations of a thin circular cylindrical membrane are produced in sucha way that the cross-sectional surface bordered by the membrane is thenaccurately circular and therefore at a maximum when the instantaneousvalue of the amplitudes of vibration passes precisely through zero.Every deviation caused by a vibration reduces the internal cross-sectionof the stencil, as a result of which either the gas located in theinterior of the stencil is compressed, or this gas, due to a slight risein pressure, is caused to flow more quickly out of the stencil throughbores which have already been opened. Energy is extracted from thevibration process thereby, which means that vibrations of the thinmembrane are damped by the gas cushion in the stencil interior.

This statement does not, however, hold for flexural vibrations of thestencil, in the case of which the individual cross-sections of thestencil remain circular. These vibrations are counteracted by theinternal material damping. In addition, as a consequence of the largeplanar moment of inertia of the stencil cross-sections and the slightmass of the stencil, the frequency of the natural flexural vibrations isso high from the start that its form of vibration is scarcely excited.

In the case of a stencil on which an internal pressure acts, theremaining radial eccentricities will be established only as those whichare conditioned by the bending of the stencil axis, i.e., theeccentricities of the individual cross-sections of the stencil. Duringengraving of the real stencil, these eccentricities cause geometricalerrors in the pattern image generated, i.e., deviations in the positionand the form of the pattern image with respect to one which would beengraved on an ideal, concentrically running and accurately circularcylindrical stencil. These geometrical errors consist, firstly, of adisplacement of the position of the engraved image in thecircumferential direction, and, secondly, of a change in width of theengraved lines generated. The latter are caused by radial distancechanges in the stencil wall with respect to the optical system whichfocuses the laser beam.

On the basis of the designations of FIG. 2, it follows from equation (5)for the distance change in the stencil wall of the real stencil bycomparison with the ideal stencil at the location of the sensor that

    ΔS.sub.sens =s.sub.0 -s.sub.sens (φ)=c.sub.1.cos(φ-ε.sub.1)                (6)

As may be seen from equation (6), Δs_(sens) can also assume negativevalues. If the stencil has rotated further by the angle Γ by comparisonwith the instantaneous position shown in FIG. 2, this is also the changewhich must be impressed on the optical system in order for the distancebetween the optical system and the stencil to remain constant and nodepth of focus errors can arise. From a direct geometrical considerationwith the aid of FIGS. 1 and 2, the radial distance change is defined as

    Δs.sub.sens =e.cos(φ-ε.sub.1)            (7)

The result for the displacement of the wall of the actual stencil in thecircumferential direction by comparison with the ideal stencils is

    Δt.sub.sens =e.sin(φ-ε.sub.1)            (8)

It is seen from a comparison with equation (6) that the eccentricity ecorresponds to the amplitude c_(i). It is necessary to correct bothincorrect positions for a geometrically correct pattern application. Theangle ε₁ also appears in addition to the angle φ of rotation inequations (7) and (8). This angle ε₁ designates the phase position ofthe maximum of the first series term in the original Fourier expansion(4) and (5). This corresponds to the angular position of theeccentricity of the real stencil cross-section with respect to the zeromark.

Equations (7) and (8) also show that the fluctuation in the pattern inthe circumferential direction leads, with respect to the fluctuation inthe distance of the stencil wall in the radial direction, by an angle ofrotation of 90 degrees, i.e., a N_(um) /4 encoder pulses. This meansthat the value of the pattern fluctuation in the circumferentialdirection was already filed earlier by N_(um) /4 encoder pulses,specifically in a memory whose address is smaller by precisely thisnumber. Considering that at the engraving point, the same distancefluctuations occur in the case of the optical system as in the case ofthe sensor, only phase-shifted by the angle Γ of rotation, it ispossible to set up the following rule in order to prevent engravingerrors.

In order to compensate the errors in the pattern image of a stencilwhich rotates with an eccentricity e but is otherwise circularlycylindrical, the radial distance s_(sens) is measured by means of aranging sensor. Furthermore, the difference value Δs_(sens) =s₀-s_(sens) is formed and this value is filed in the memory location k asΔs_(sens) (k). After a further

    N.sub.Γ =N.sub.um.Γ/2.π                     (9)

pulses received from the encoder, this value is read out again from thememory location k and is impressed on the optical system, which focusesthe laser beam, as a radial actuating pulse. This state of affairs canbe represented by the following symbolic rotation:

    Δs.sub.opt <k+N.sub.Γ >=Δs.sub.sens (k)  (10)

The notation employed is intended to signify: extract the contentΔs_(sens) (k) from the memory location k upon receiving the <k+N.sub.Γ >-th pulse, and impress it on the optical system as actuator travel.

Furthermore, likewise upon receiving the <k+N.sub.Γ > -th pulse, thecontent of the memory location with the address k-N_(um) /4 (thiscontains the value Δs_(sens) stored earlier by a quarter turn), isextracted and used to determine the pattern displacement in thecircumferential direction. This is described symbolically by

    Δt.sub.opt <k+N.sub.Γ >=Δs.sub.sens (k-N.sub.um /4)(11)

Now equation (11) is used to calculate the corrected address of thememory location for the pattern information. If the value Δs_(sens)(k-N_(um) /4)>0, the retrieval of the pattern information must lead,i.e., the pattern information provided at other memory locations for thepurpose of controlling the laser beam must be extracted from a memorylocation whose address is higher by the integer ##EQU4## than the memorylocation number for the ideal stencil. In equation (12), the valueΔt_(opt) corresponds to the value of the relationship in equation (11).If, inversely, the value Δs_(sens) (k-N_(um) /4)<0, the memory addressread out for the purpose of clearing the pattern information must besmaller by the amount, which follows from equation (12), than the memorylocation number would be for the ideal stencil.

A further short comment on the use of the manipulated variabledetermined for the optical system is provided below. The radialpositional error of the stencil wall is compensated owing to the factthat, in accordance with the manipulated variable Δs_(opt) determined,the optical system is adjusted in the direction of its optical axis. Inthe case of the relationships considered here, this is, however, arelatively slow movement, since it was, after all, assumed that only theerror of first order remains thanks to the application of internalpressure to the stencil. The optical system must then be moved only at afrequency corresponding to the rotational speed of the stencil, and thespeed and frequency are 15 Hz for a rotational speed of the stencil of900 rev/min. Of course, the pattern position could likewise be correctedin the circumferential direction by adjusting the optical system.However, preference is given to the later or earlier retrieval, possiblewithout inertia, of the pattern information from the semiconductormemory of the computer.

Measuring and processing the radial eccentricity in the case of anunpressurized stencil which is not exposed to the excitation ofvibrations:

Consideration will now be given to the relationships in the case of astencil which is held at both its ends by two accurately concentricclamping devices, and to which an excess pressure is not applied on itsinside. The rotary actuator of the stencil is to be constructed to beentirely precision balanced, for example owing to the fact that thedrive is performed via damping V-shaped or flat belts, and that also allthe belt pulleys and shafts are very well balanced and the driving motoroutputs a very uniform torque, with the result that no membranevibrations of the stencil wall are to occur.

In the case of such a stencil, it is necessary to expect radialeccentricities whose Fourier representation still contains substantialvalues for the series terms of higher order by comparison with a stencilsuch as has already been treated above. All these radial eccentricitiesare of a static nature, i.e., they rotate with the stencil, or, in otherwords they do not change in a notional coordinate system permanentlyconnected to the stencil.

Once the compensating paths have been determined, the compensation thenrequires very quickly reacting actuating mechanisms, such as actuatingelements which are based on piezoelectric effects, because the frequencyof the compensating movement must correspond to a value which is equalto the rotational speed multiplied by the order of the respectiveFourier term. This holds, in particular, for compensating the incorrectradial position of the stencil wall. The incorrect position of thepattern in the circumferential direction can be corrected again with theaid of the inertialess method of later or earlier retrieval of thepattern information.

The first step is to determine the two correction signals. To calculatethe incorrect position of the actual stencil, an attempt can be made todetermine the arc length of the real stencil shown in FIG. 3, and torepresent it in analytical form, by using equation (4). It is known tohold for the arc length in polar coordinates that ##EQU5## Here, dr/dφcorresponds to the differentiation of equation (4) with respect to φ,and because of the generally small errors it was assumed that ##EQU6##It follows for dr/dφ from equation (4) that ##EQU7##

Substituting this into equation (13) leads to complicated expressionsfor the arc length b even in the case of a short Fourier series, andthus requires the determination of an analytical relationship for thepattern displacement, and its numerical evaluation certainly requires avery high outlay on computation to be applied.

A different mode of procedure is better. The value of s_(sens) (k) ismeasured in the case of each received encoder pulse. This value is usedto form Δs_(sens) (k)=s₀ -s_(sens) (k), as already described above, andthese values are stored. The length of the segment of the stencilcircumference situated between these measuring points can then becalculated from two successive measured values Δs_(sens) (k) andΔs_(sens) (k+1) as ##EQU8##

(15) Summing these sublengths from the measuring point marked by thezero mark signal up to the k-th measuring point yields ##EQU9##

The desired circumference of the ideal comparison stencil up to the samek-th measuring point is yielded as ##EQU10##

The difference between these two values should correspond to thedisplacement error in the pattern in the circumferential direction.Since, however, there are always influences which cause substantialcircumferential differences, for example thermally conditioned changesin position of the sensor or a slightly larger stencil diameter or thepolygonal effect conditioned by the division of the stencilcircumference into N_(um) measuring points, it is expedient todetermine, after each complete rotation of the stencil, a correctionfactor which is available during the following rotation for thecorrection of the subtraction. The correction factor is determined from##EQU11## and is used for calculating the displacement error in thecircumferential direction as follows:

    Δt.sub.sens (k)=Umf.sub.des (k)-K.sub.corr Umf.sub.act (k)(19)

The two value sequences Δs_(sens) (k) and Δt_(sens) (k) for each of theincoming pulses (from 0 to N_(um)) are now stored and the correctionvalues are formed as follows, using the symbolic notation alreadyemployed in equations (8) and (9),

    Δs.sub.opt <k+N.sub.k >=Δs.sub.sens (k)        (20)

    Δt.sub.opt <k+N.sub.Γ >=Δt.sub.sens (k)  (21)

The rules to be applied here for avoiding engraving errors are thus

In the case of the k-th incoming encoder pulse form the value Δs_(sens)(k)=s₀ -Δ_(sens) (k), and store the latter at the memory location of thevalue sequence Δs_(sens) with the address k. In the case of the<k+N.sub.Γ > -th pulse, clear the value from the memory location k andimpress this value on the optical system as a radial actuator travel.

In the case of k-th incoming encoder pulse, form the value Δt_(sens) (k)in accordance with the relationship in equation (19), and stored that atthe memory location of the value sequence Δt_(sens) with the address k.In the case of the <k+N.sub.Γ > -th pulse, clear the value from thememory location k and either impress this value on the optical system asa tangential actuator travel or use the relationship in equation (10) tocalculate an integral value by which the address of the memory locationof the pattern information is to be increased if the value s₀ -Δs_(sens)(k-N_(um) /4) is smaller than s₀, but if this value is greater than s₀the address of the memory location of the pattern information is to bediminished by the value calculated from the relationship in equation(10).

Measuring and processing the radial eccentricity in the case of anunpressurized stencil which is excited by external forces to vibrationof the thin hollow cylinder envelope.

The last case to be considered is a stencil to which a pressure is notapplied on its inside, and which is excited to vibration by temporarilyvarying external forces. In this case, a first ranging sensor (stillfurther sensors have to be introduced later) measures a distance for thetime-dependent changes on which two different causes are decisive. Thefirst cause is the static radial eccentricity. This is present as afunction of χ over the circumference of the stencil and is defined byequation (4). The angle χ is measured starting from a zero mark providedon the stencil circumference, specifically in a polar coordinate systemwhich is permanently connected to the stencil and whose origin lies onthe stencil axis. This angle is to be counted positively in thatdirection which opposes the direction of rotation. Since the stencilrotates at the angular velocity ω, the static radial eccentricities moveFast the sensor at the circumferential speed R·ω. Furthermore, anangular position φ of the stencil is to be defined with respect to theposition of the first sensor, specifically most effectively by measuringthis angle between the zero mark and the first sensor.

It then holds for φ

    φ=ω·t                                   (22)

This relationship also defines the instant t=0. The latter is determinedowing to the fact that the zero mark moves precisely past the positionof the first sensor. The distance changes previously described aresuperimposed by distance variations which are to be ascribed to thevibration of the stencil, and this is the second cause for thetime-variable distances measured by the sensor. The time dependency ofthis second component of the distance change is also the reason whichprompts including the time t in the considerations here.

In order not to complicate the relationships excessively, it is to beassumed that the thin stencil vibrates at only one frequency. Moreover,the maximum value of the amplitude of vibration is to remain constantduring the observation period. This case will be the most important casein practice, because it is to be assumed that the excitation ofvibration is caused by the rotary movement of the stencil and that thevibration frequency therefore corresponds to the rotational speed or amultiple thereof. The ever present material damping effects a stationaryvibration state after a short settling time. It is therefore necessaryto add a further vibration component to the distance changes on thebasis of the static radial eccentricities (relationships set forth inequations (4), (5)). It then holds for the radial eccentricity measuredby a first sensor that: ##EQU12## The term

    c.sub.dyn.sin(n.φ+Φ).cos(κ.t+θ)        (24)

added to the relationship of equation (5) consists, as is known in thecase of vibrating continua, of three factors, specifically an amplitudec_(dyn) of the position function sin(n.φ+Φ) and the time-dependentfunction cos(κ.t+θ). Here, c_(dyn) signifies the maximum amplitude ofvibration of the stencil, i.e., the amplitude at the points of theantinodes. The number of the antinodes along the circumference of thestencil, sometimes also designated as the order of the vibration, is n.The unknown angular distance between the nearest node on the stencil andthe zero mark is θ. The unknown time interval T₀ of the last maximumamplitude of the vibration from the instant t=0 converted into a phaseangle is θ, and it holds that θ=T₀ ·κ. Finally, κ signifies the angularfrequency of the vibration.

It is expedient for understanding the additive term set forth inequation (24) to adopt an observation standpoint fixed in the sensor andto regard the vibration picture of the stencil (that is to say, itsantinodes and nodes) in its circumferential position as if it werepermanently connected to the remaining form errors of the stencil.However, in contrast to the static form errors, whose size is constantwith time, the antinodes pulsate in time with the vibration frequencyand at the same time this pulsating feature moves past the sensor.

Yet a further remark on the order n of the vibration. This order nrefers to the number of antinodes on the circumference of the stencil;the antinodes in the axial direction are of no interest here. The ordern=1 signifies that only a single antinode occurs on the circumferenceand rotates with the stencil. This corresponds to a flexural vibrationof the stencil, such as can be observed, for example, in the vicinity ofa critical speed. It is known from mechanics that the antinode rotateswith the rotation of the shaft in the case of flexural vibration, aswell. For n=2, the stencil vibrates in such a way that an oval is formedas vibrational form, and for n=3 the vibrational form is a triangle. Ifthe stencil vibrates at n=2 as an oval, an observer rotating with thestencil system would see a ring which has the larger diameter now in onedirection and, in the next half period, in the direction perpendicularthereto. Of course, a ranging sensor measures only a variable distanceto this rotating and simultaneously vibrating feature.

Generally, the stencil must not have maximum deflection precisely whenthe antinode passes by the sensor. For this reason, it may be expectedthat it is not possible using a single sensor to detect the vibrationstate of the stencil, and certainly not possible to separate said statefrom the form errors. The same conclusion can also be reached byconsidering the relationship in equation (23). Although after ameasurement of a complete stencil circumference there are N_(um)measured values available, it is necessary to use these to determine theN_(um) coefficients a_(i) and b_(i) (i=0, 1, . . . , N_(um) /2) of theFourier series terms in equation (23). There are then no moreconditional equations remaining to determine the unknown parameters ofthe dynamic term of equation (24) will stop.

The aim is therefore to arrange a second sensor at an angle α to thefirst sensor, which detects the distances from the stencil wall inaccordance with the relationship in equation (23). An observer B2positioned on this sensor would see the same vibrating stencil, i.e.,the errors thereof, antinodes and nodes as an observer B1 on the firstsensor as shown in FIG. 3. It is now to be assumed that at an arbitraryinstant t during a rotation the observer B1 makes a recording of theinstantaneous state of the stencil and records the angular distancesfrom the antinodes and the prominent errors. If the observer B2 werelikewise to measure the instantaneous state of the stencil at the sameinstant, he would establish the same amplitudes of the antinodes, butdifferent relative angular positions to his position. Specifically, thelatter differ by the angle α from the instantaneous recording of theobserver B1. However, if the observer B2 waits for that time intervalΔt=α/ω which the stencil requires in order to rotate precisely by theangle α, the observer B2 sees the same relative instantaneous positionsof the prominent stencil errors, the antinodes and nodes, but differentinstantaneous values of the amplitudes of vibration. The latter areyielded corresponding to the relationship in equation (24) at the laterinstant

    t+Δt=t+α/ω as

    c.sub.dyn.sin(n.φ+Φ).cos(κ.(t+α/ω)+θ)(26)

The relationship in equation (23) held for the distance of the firstsensor from the stencil wall at the instant t, and the relationship##EQU13## holds correspondingly for the distance of the second sensorfrom the stencil wall in a completely analogous way, but at the laterinstant t+α/ω and where s₀ =A-R₀.

In equation (27), φ once again denotes the same angle of rotation of thestencil as in equation (23) . The relationships in equations (23) and(27) are used to elaborate those equations which are required in orderto consider whether it is already possible with the aid of themeasurement results of the two sensors to forecast accurately thedistances of the stencil wall from an arbitrary observer or component,for example the optical system. This observer is to be permanentlyconnected to a sensor system, but to adopt an arbitrarily prescribedangle to this system at the periphery of the rotating stencil.

The relationships in equations (23) and (27) are written down onceagain, but with the aid of a suitable notation the lefthand side is moreeffectively stressed, so that we are dealing with two differentdistances which are measured by two different sensors at two differentinstants. ##EQU14## If the difference between these measurement resultsis now formed, and the angular coordinate φ is introduced in accordancewith equation (22) instead of time (t=φ/ω), the static error componentsare eliminated by subtraction, and the following is obtained for thedifference signal ##EQU15## An attempt must now be made to determine theparameters of this relationship which characterizes the dynamic errorcomponent. In order to display more effectively the physical import ofthe relationship in equation (28), two auxiliary variables ε and η areformed for which ##EQU16## It then follows from equation (28) that##EQU17## and it is to be seen from this form of the difference betweenthe two sensor signals that the difference is composed of two sinesignals having an amplitude of the same size but being of differentfrequencies, i.e., a beat response. The unknown parameters in therelationship (30) and, of course, also in equation (28) are thevariables C_(dyn), κ, ω, θ, n and Φ.. The auxiliary variables ε and η,by contrast, can be ascribed to κ, ω and the known angular distance αbetween the first and second sensors. It holds that ##EQU18## it is alsopossible to find a direct functional relationship for equations (31a)and (31b) by eliminating parameter ##EQU19## Recourse will be made tothis later. ##EQU20## The special form of the difference signaldescribed by equation (30) suggests it is expedient to express thisequation in a form which permits simple comparison with a Fourierseries. For this purpose, ##EQU21## is formed, ##EQU22## is substitutedtherein, and ##EQU23## is obtained in the desired Fourier form. It isseen that only two terms of this series stand out. The first term isthat of order n-κ/ω. This can be of zeroth order if n-κ/ω=0. Thequotient κ/ω is formed from (the angular frequency κ of the membranevibration and from the angular velocity (ω) of the stencil. Since thevibration is excited by the rotary movement of the stencil, only integerratios κ/ω occur, and the orders n-κ/ω and n+κ/ω then likewise remainintegers.

If the values of the coefficients a_(low), b_(low), a_(high) andb_(high) were known in conjunction with the parameters n, κ and ω inequation (32) , it would be possible to determine the value of thedifference signal D₂₁ for each arbitrary angle φ. Although,unfortunately, coefficients and parameters are not known, the value D₂₁=Δs_(sens).1 -Δs_(sens).2 stored in each case for one rotation areavailable. These can be used to recalculate the coefficients a_(low), .. . , b_(high), for example using the known method of fast Fouriertransformation (FFT). Once the coefficients have been determined, fourequations (31a-31d) are available for the purpose of calculating furtherunknowns of the six parameters c_(dyn), ε, η, Φ, n and θ. Apart from thecoefficients mentioned, in the case of FFT, the orders of thesecoefficients also occur, that is to say

    01=n-κ/ω                                       (34a)

    02=n+κ/ω                                       (34b)

because the coefficients of all other orders vanish.

The value of Φ can be determined very simply: ##EQU24## Adding these twoexpressions

    a.sub.low +a.sub.high =c.sub.dyn.η.sin(Φ).cos(θ-ε); (35)

is obtained; it follows analogously that:

    b.sub.low +b.sub.high =c.sub.dyn.η.cos(Φ.cos(θ-ε); (36)

and, finally, from equations (33) and (34) that ##EQU25## It may beshown, furthermore, that ##EQU26## holds for the difference θ-ε

The value α.κ/ω can be eliminated from equations (29a) and (29b) and thefollowing is obtained for ε as a function of η: ##EQU27## Finally, itcan be shown with the aid of equation (32) that ##EQU28## must hold.

The expressions (38), (39) and (40) represent three equations fordetermining the four unknowns θ, ε, η and c_(dyn). The system ofequations is therefore not sufficiently determined and no solution isyet possible. This could already have been realized from equation (34),because this system of equations was already underdetermined and, inaddition, the variables Φ and ε and likewise the variables c_(dyn) and ηalways occurred in the same combination in equation (34).

A solution of the present problem therefore requires the mounting of athird ranging sensor which is offset relative to the first rangingsensor by the installation angle β, there being a requirement that β≠α.Only with the measured values additionally supplied by this third sensordoes it become possible to determine all the variables required todetermine the stencil vibration, and in this way to separate the radialeccentricities as a consequence of vibration from the static radialeccentricities. This is necessary because the calculation of the radialand tangential positional errors at the location of the laser beam canonly be performed separately for the static eccentricity and thevibrational deviation.

However, a transformation of equation (40) is required as a first step.From equation (40) it follows, if η is replaced by its equivalent fromequation (31b) and it is considered that 1-cos(x)=2.sin² (x/2) , that##EQU29## In a manner entirely similar to how it was demonstratedpreviously for the difference signal of the second and first sensor, theresult for the difference signal between the third and first sensor isobtained, instead of equation (34), as ##EQU30## Values a_(low),b_(low), a_(high) and b_(high) are calculated, in a similar way asalready done earlier for the coefficients a_(low), b_(low), a_(high) andb_(high), with the aid of an FFT of the value sequence, determined bymeasurement, for D₃₁. It holds for these values by analogy with equation(39) that: ##EQU31## It may be recalled that c_(dyn) is the maximumamplitude of vibration of the stencil at its circumferential antinodes.The amplitude therefore does not depend on the position of a sensor. Ifβ=2α is now further chosen, it follows from equations (41) and (43)that: ##EQU32## and, further, that ##EQU33## and finally ##EQU34## isobtained.

This relationship (44) is the key to the further evaluations, and nowfinally permits the determination of κ/ω. If this value is known, it ispossible to draw conclusions on variable ε by means of equation (31a),and on the value of η by means of equation (31b). The variable Φ followsfrom equation (38) with a known ε and, finally, the value of c_(dyn)follows from equation (40) with a known η. It is also possible todetermine the number of the antinodes n, where n is the order of thevibration, from equations (34a) or (34b). All the parameters which wereplaced as unknown in the vibration component of the relationshipequation (23) are now determined.

The values s_(sens).1 (φ) measured by the first sensor during a rotationcan therefore be freed from their vibration component (24). The correctdetermination of φ and t is to be borne in mind here. In the positionfunction sin(n.φ+Φ) of the vibration component (24), φ denotes the anglebetween the zero mark and the first sensor at the measuring instant t.The result for the distance from the stencil wall to the sensor which isfreed from the dynamic component is ##EQU35## Consequently, it followsfor the static part of the difference of the measured distances from thesensor to the ideal stencil and from the sensor to the real stencilΔs_(sens).1.static =s₀ -s_(sens).1.static (φ), where s₀ =A-R₀, thus that##EQU36##

The further calculation can be better surveyed if φ in equation (47) isreplaced by the number of the encoder pulses which are counted startingwith the zero pulse. It then follows that ##EQU37## and this valueΔ_(sens),1,static is stored at the memory location k of its measuredvalue sequence. If the stencil position measured by the sensor islocated below the optical system--a further N_(um) ·Γ/(2·π)=N.sub.Γpulses have then elapsed--this value is cleared, corrected by thevibration component then valid, and finally, impressed on the opticalsystem as a radial manipulated variable. This state of affairs isdescribed as follows with the aid of the symbolic system already usedearlier ##EQU38## It is important for the purpose of calculating thepattern displacement in the circumferential direction to know whetherthere is an envelope generated which nowhere experiences a displacementin the circumferential direction. FIG. 3 shows a circular cylindricalstencil which vibrates in the form of an oval. Given small amplitudes ofvibration, the amplitudes are equal in both directions leading out ofthe equilibrium position in the case of all vibrating bodies.

It then follows from a simple geometrical estimation of the arc lengthsbetween the nodes that

firstly, the node must experience a deflection in the circumferentialdirection which is equal to the deflection of the antinodes in thedirection normal to the membrane surface, and that

secondly, for reasons of symmetry, the displacement of the antinodes ofthe membrane vanishes in the circumferential direction.

An envelope generated by the stencil through an antinode (parallel tothe axis) thus exhibits the desired response. Because of the randomnature of the selection of the zero mark, it is not possible to expectthis response of the envelope generated at this point. It would bepossible, after all, for the zero mark to be situated precisely at thepoint of an antinode.

The calculation of the displacement is therefore to proceed from such anantinode. An antinode adjacent to the zero mark must be situated wherethe position function sin(2·n·k·π/N_(um) +Φ) in equation (23) assumesthe value 1. It follows that ##EQU39## and the pulse number k_(B)=(pulse number from the zero mark up to the first antinode position)corresponding to the antinode position thus becomes ##EQU40##

The next calculation step consists in determining the circumferentialsublengths db(k) of the stencil between in each case two measuringpoints, and to sum these, starting from the position of the antinode, upto the engraving point, in order to obtain the arc length of thispartial circumference of the stencil. It is then necessary to use thepartial circumference to form the distance relative to the correspondingpartial circumference of the ideal stencil. This is the displacement ofthe stencil wall in the circumferential direction. The sublengths db(k)are to be geometrically composed, as already earlier, from radial andtangential segments. The radial segments consist here of a static and adynamic error component. The result for the static error component ofthe radial segment, as shown in FIG. 4, is

    db.sub.rad stat (k)=Δs.sub.sens,1,sens (k)-Δs.sub.sens,1,sens (k+1)                                                     (52)

The dynamic error component is to be ascribed to the vibration,inserting the engraving time t_(G) in the time function ZTF of all thesegments for t. ##EQU41## The following is obtained for the tangentialsegment ##EQU42## The geometrically combined sublength becomes ##EQU43##

The summation of these sublengths is best carried out at first for acomplete circumference, in order to be able to determine a correctionfactor K_(corr) already used earlier. This is also intended here tocompensate effects such as those of sensor displacement or the polygoneffect. The result ##EQU44## It now further follows for the arc lengthsof the partial circumference of the real stencil that ##EQU45## and forthe arc length of the partial circumference of the ideal stencil that##EQU46##

Finally, it is possible from this to determine the circumferentialdisplacement of the pattern image at the point of the k-th pulse, but atthe time of the transmission of the (k+N.sub.Γ) -th pulse at theengraving point. The result is

    Δt.sub.opt <k+N.sub.Γ >=Umf.sub.des (k)-K.sub.corr UMF.sub.act (k)                                                       (60)

The computational requirements for determining the corrections Δs_(opt)and Δt_(opt) appears complex. The complexity of the calculation is,however, of interest only for setting up the program. More importanceattaches to the computational requirements for the program whilerunning. There is a need to determine whether such requirements for agiven computer performance are acceptable. It is to be pointed out herethat only very slight differences are to be observed in the magnitude ofthe amplitude of vibration over length sections of the stencil of theorder of magnitude of approximately 10 mm, and that the displacementsΔs_(opt) and Δt_(opt), once calculated within these length sections,hold for all the series. The vibrational form, i.e., the order n of thevibration, is even preserved over the entire length of the stencil andchanges only if the exciting frequency (rotational speed) changes.Assuming that the width of an engraving line amounts at most to 100 μm,the computing run described has to be carried out only once forapproximately 100 rotations of the stencil, a computational outlay whichcan be justified.

A device according to the invention for carrying out the methoddescribed above is shown in FIG. 5. A screen printing stencil bears thereference symbol 1, while a hollow cylinder is provided with thereference symbol la. Located on the hollow cylinder 1a is a stencilpattern 1b, specifically inside a light responsive layer, i.e., a layerfor which at least one physical characteristic is altered when lightimpinges upon it, preferably a lacquer layer 1c, which is situated onthe outer circumferential surface of the hollow cylinder 1a. In thiscase, the hollow cylinder 1a is, for example, a uniformly perforatednickel cylinder.

The screen printing stencil 1 is held at its mutually opposite end facesby, in each case one clamping head 2 or 3, which are constructed ascentering flanges. These clamping heads 2, 3 are respectively rotatablymounted in a bearing shell 4, 5. The bearing shells 4, 5 are supportedon a machine bed 6, specifically via support devices 7, 8.

The support device 8 can be removed from the machine bed 6, or can bedisplaced relative to the machine bed 6 in the longitudinal direction ofthe screen printing cylinder 1a so that the cylinder 1a can more easilybe positioned between the clamping heads 2,3 or can be taken out againfrom the region situated between them.

A hollow shaft section 9 connected to the left-hand clamping head 2extends into the bearing shell 4 and is rotatably mounted there. Thishollow shaft section 9 is set rotating via a drive chain which runsthrough the support device 7 up to a drive motor which is arranged inthe machine bed. When the hollow shaft section 9 rotates, it drives theclamping head 2, with the result that the screen printing stencil 1 isthereby set rotating. The other clamping head 3 rotates freely and ismounted in the bearing shell 5 via a hollow shaft section 10.

The two hollow shaft sections 9 and 10 end in the region of the clampinghead 2 and 3, respectively, i.e., they do not extend into the screenprinting stencil 1, and are, furthermore, connected sealingly to flowducts 11 and 12 at their ends away from the clamping heads 2, 3.

A shaft encoder 13, connected to the free end of the hollow shaftsection 9, informs, via a control line 14, a computer 15 with anassociated monitor 16, of the respective rotary position of the screenprinting stencil 1. In this case, the computer 15 sends appropriateswitch-on or switch-off pulses to a laser 17 via a control line 18. Alaser beam 19 of the laser 17 is emitted or not emitted in accordancewith these switch-on and switch-off pulses, respectively. A firstdeflecting mirror 20 directs the laser beam 19 to a second deflectingmirror 21. The second deflecting mirror 21 is mounted together with afocussing lens 22 on an optics carriage 23 which is movably arranged onan adjusting carriage 23a. As will be explained later, the adjustingcarriage 23a is supported indirectly on the machine bed 6, which standson the ground, for example, just like a post 24 for holding the firstdeflecting mirror 20.

In the region between the first deflecting mirror 20 and the seconddeflecting mirror 21, the laser beam 19 extends parallel to the cylinderaxis 25 of the screen printing stencil 1 and is deflected by the seconddeflecting mirror 21 in such a way that it impinges at leastapproximately radially on the hollow cylinder 1a. In this case, it isfocussed onto the lacquer layer 1c by means of the focussing lens 22.

The adjusting carriage 23a can be displaced in the direction of thecylinder axis 25 of the screen printing stencil 1. This displacement iseffected by a spindle 26 and a motor 27 which drives this spindle. Around guide 28 and a prismatic guide 29 ensure a movement of theadjusting carriage 23 which is exactly parallel to the cylinder axis 25of the screen printing stencil 1. In this case, the prismatic guide 29is located on the upper surface of the machine bed 6, while the spindle26 and the round guide 28 are arranged parallel to one another on thefront of the machine bed 6.

Located inside the machine bed 6 are gas delivery devices of whichrespectively one is connected to one of the flow ducts 11 and 12. Acompressed gas can be blown into the interior of the screen printingstencil 1 by these gas delivery devices via the flow ducts 11, 12, thehollow shaft sections 9, 10 and the clamping heads 2, 3. A sealing meanscan also be blown with the gas into the interior of the screen printingcylinder 1, in order, if necessary, to seal off the inside openings,freed from the lacquer layer 1c, in the hollow cylinder 1a. The sealingmeans can be, for example, material chippings, for example paper shredsor small plastic discs and the like, which can also have a reflectingsurface.

The motor 27 for driving the spindle 26 is preferably a stepping motor,with the result that it is also possible to determine the axial positionof the laser beam 19 impinging on the hollow cylinder 1a by means of thedrive pulses for the stepping motor 27. Corresponding drive pulses arereceived by the stepping motor 27 from the computer 15 via a line 27a.

A bow 30 is connected to the adjusting carriage 23a, for example in onepiece, and comes to lie below the screen printing stencil 1 andsurrounds the latter at a distance, for example in the shape of acircular arc or semicircle. The bow 30 is thus correspondingly moved insympathy with the movement of the carriage 23a in the direction of thecylinder axis 25. Attached to the bow 30, or recessed therein, are oneor more ranging sensors 31 which are aligned radially relative to thescreen printing stencil 1 and measure the distance between them and thesurface of the screen printing stencil 1 or the hollow cylinder 1a. Thedistance-measuring signals are sent via a line 32 to the computer 15.

The bow 30 can carry, for example, three ranging sensors 31 which aresituated at different circumferential positions on the screen printingstencil 1. These ranging sensors 31 are used to measure, at respectivelyfixed measuring positions, radial deviations in the actual position ofthe wall of the hollow cylinder 1a from its ideal position for amultiplicity of circumferential positions of the hollow cylinder 1a whenthe cylinder 1a is rotating.

The respective measuring signals are then processed in the computer 15,in order to determine the radial positional deviations from the measureddistances between the sensor and the hollow cylinder surface. The firstand second actuating signals already mentioned at the beginning areobtained therefrom. The first actuating signal is used for the purposeof retrieving the pattern information sooner or later from a storagedevice located in the computer 15. As a result, the tangentialdisplacements of the hollow cylinder wall can be compensated. The focusof the laser beam is held permanently in the region of the lacquer layer1c by the second actuating signal, and this can be performed byappropriate displacement of the optics carriage 23, which can bedisplaced relative to the adjusting carriage 23a and in the radialdirection of the screen printing stencil 1. The displacing signal fordisplacing the optics carriage 23 is fed to the latter from the computer15 via a line 33.

Unfortunately, thin-walled screen printing stencils are not idealcircular cylindrical bodies. Both along their longitudinal extent and intheir cross-section, they have deviations from the ideal shape of thecircular cylinder which prevent the pattern from being applied in apattern-to-fit fashion and require that the effects of these deviationsbe rendered harmless, for example by measurement and the introduction ofsuitable correction measures.

FIGS. 6 and 7 show a screen printing stencil 1 of non-ideal, circularcylindrical shape. FIG. 6 shows a typical cross-sectional variation andFIG. 7 shows the deviations in the longitudinal section of the screenprinting stencil 1. The radial eccentricities 34 of the screen printingstencil 1 are represented by comparison with an ideal circle 35, theseeccentricities having been drawn, for reasons of clarity, somewhatlarger in relation to the errors occurring in practice. Three sensors31a, 31b, 31c are provided for the purpose of measuring these radialeccentricities 34. Such sensors are known and can establish the distancechanges from a reference point on the basis of inductive, capacitive oroptical effects. They are all aligned on the cylinder axis 25.

As already explained at the beginning, the radial eccentricities 34 havea plurality of causes. Firstly, there are static form variations. Thisterm refers to the deviations, already mentioned, from the accuratecircular cylindrical shape which can already be observed in the case ofa stencil at rest. These are to be ascribed, for example in the case ofa thin-walled nickel stencil produced by electroforming, to internalmaterial stresses which occur more or less by accident during theprocess of manufacturing such a stencil. It is possible in the case ofthese static form variations to distinguish deviations of thecross-section from the circular shape 35 and deviations of the realstencil axis 25a from its ideal straight-line extent 25. In the case ofa cross-section which is exactly circular, this last named formvariation produces a radial eccentricity 34 as a consequence of aneccentricity of the center of the circle. In addition to the static formvariations, there are ones which are dynamically conditioned, i.e.,those which are caused by vibrations of the thin stencil wall.

In order to detect all these radial eccentricities, three rangingsensors 31a, 31b, 31c are used, and these are, moreover, arranged withrespect to each other at two different angular distances α and γ, whereα+γ=β. This is very advantageous for separating the signals in staticand dynamic components. The sensors 31a, 31b, 31c are provided in amounting position in which they are no longer influenced by othercomponents, i.e., ferromagnetic materials are, however, not provided inthe environment of the active end of an inductive sensor. The sensorsare preferably arranged in the lower cross-sectional half of the stencil1 directly adjoining the engraving point 36. The sensors are held in thestiff bow 30, which is not susceptible to vibration and is permanentlyconnected to the adjusting carriage 23a. This adjusting carriage 23acarries an optics carriage 23, and the latter in turn carries the lens22 in a lens mount 22a.

Not only can the radial eccentricities 34 be measured and adjusted bymeans of the optical system in agreement with the measured results, butalso the radial eccentricities 34 can be kept small from the very first.For this purpose, air at a slight overpressure is blown into theinterior of the stencil 1 starting from the end faces thereof. Theeffect of the membrane stresses produced in the stencil envelope is thatthe latter is stressed outwards in a largely circular fashion. Since theremaining radial eccentricities are rendered small to very small by thismeasure, it is also possible for the measuring range of the sensors 31a,31b or 31c to be selected to be small, resulting in an even moreaccurate measurement result than would be the case without theapplication of pressure to the inside of the stencil.

In process steps following the measurement of the radial eccentricity,the measuring signals of the three sensors 31a, 31b, 31c obtained duringa rotation are decomposed into their components to be assigned to staticand dynamic form variations. The coefficients of a first order areobtained from these components which are formed for example with the aidof FFT (Fast Fourier Transformation), and the actuating signals for thefocal point are derived from these terms. Since, in accordance withexperience, the vibrations of the stencil wall have small to very smallamplitudes, particularly when the wall is subjected to an internaloverpressure, even if only slight, the actuating signals obtained fromthe FFT can be used for a plurality of successive rotations of thestencil. It is also possible, as the case may be, to avoid the FFT byusing RC filtering to obtain the signal component of the first orderfrom the analog measuring signals immediately downstream of the sensors.In particular cases, this means that given effective rounding out of thestencil by a sufficiently high internal pressure, it is also possible touse the unfiltered signals instead of the signal component of the firstorder.

Once the actuating signals have been determined, the focus is adjustedin a different way. Firstly, the focus is adjusted in the radialdirection by radial movement of the optical system, that is to say thelens 22. Then the focus is adjusted in the circumferential direction ofthe stencil 1. The most favorable method here is to leave the focalpoint always at the same physical position, but to provide the patternsignal, somewhat sooner or somewhat later via the computer, so thatengraving is always carried out correctly despite the displacement ofthe circumferential position of the stencil. Thus, if the wall of thestencil has been somewhat advanced with respect to the desired position,the pattern signals are transmitted to the laser by the computersomewhat sooner and vice versa.

A possible way of adjusting an optical system 37 is shown in FIGS. 8 and9. This system consists of the focussing lens 22, which is held by alens mount 22a, and a moveable deflecting mirror 38. The lens mount 22ais held by leaf springs 39 which permit backlash-free movement of thelens mount 22a in the direction of the optical axis 40, but support thelens mount 22a in a relatively stiff fashion in all other directions.The leaf springs 39 are provided in the form of sets and it is possibleowing to the dimensioning of the leaf springs to keep the displacementof the focussing lens 22, and thus of the focal point, within requiredtolerance limits in all directions perpendicular to the optical axis 40.

A positioning motor 41 ensures the movement of the lens 22 and of thelens mount 22a in the direction of the optical axis 40. In principle,the positioning motor 41 exhibits the design of a plunger coil. Thecontrol of the position of the lens 22 can be performed either bymeasuring the voltage across the coil 42 or by means of a ranging sensorknown per se (not further represented), which continuously measures theposition of the lens 22 or of the lens mount 22a permanently connectedthereto. In the case of measurement of the coil voltage, the position isdetermined by means of the coil force, which increases with risingvoltage and then deflects the leaf springs 39 every more strongly. It isalso possible to provide a further spring, for example a preloadedhelical spring, as an element amplifying the counterforce, so that theworking point of a device can be kept as near as possible to thestretched position of the leaf springs 39, and also the tendency of thearrangement to vibrate can be kept low by the increase in the springconstants.

The control of the displacement of the focal point in thecircumferential direction of the stencil 1 is performed by thedeflecting mirror 38 which is supported on two precision ball bearings43. The axis of rotation of these bearings 43 agrees with the beam axisof the beam 19 emitted by the laser 17. The direction of the beam guidedby the lens 22 can be varied by swivelling the deflecting mirror 38, andthe focal point can thereby be displaced on the stencil 1. Theswivelling movement is initiated here by a piezoelectric actuatingelement 44 which acts with its upper end via a leaf spring 45 on a lever46 of the deflecting mirror 38 and swivels the deflecting mirror 38 inthe event of length changes in the actuating element The deflectingmirror 38 is supported on the adjusting carriage 23a via a bearing bolt47 and a bearing block 48. The fixed lower end of the piezoelectricactuator 44 is stiffly connected to the adjusting carriage 23a. Thisadjusting carriage 23a moves on round guides 49 parallel to the stencilaxis 25.

FIGS. 10 and 11 show a similar device in which, however, the deflectingmirror 38 is carried not by ball bearings but by cruciform springs 50.This bearing is distinguished by absolute freedom from backlash and avery high accuracy. However, in this case the positioning motor 41 isreplaced by a further piezoelectric actuating element 44a with a leafspring 45a. Otherwise, the same designations apply as in FIGS. 8 and 9.

Represented in FIG. 12 is the circuit which effects a displacement ofthe pattern information in accordance with the incoming first actuatingsignal 76. A bus control logic 77 controls the first correct setting ofan address counter 78 which also controls in the further sequence thesetting of the correct address of a RAM 79. This RAM 79 is loaded via an8-bit wide data bus 82 and a switchable driver 80, with a data recordwhich contains the sequence of lengths of the pattern point intervals tobe applied to the stencil. These are not all of the same length, sinceit is to be possible for any arbitrary number of pattern points to beapplied to the circumference of the stencil, and this is possible onlyby means of a preselected sequence of slightly different intervallengths. The driver 80 can be opened or blocked via a signal line 85,and am the same instant a further driver 83, which is connected via aninverter 84 to the same signal line 85, is blocked or opened. Thisamounts to nothing other than that at an instant the RAM 79 can eitheronly be loaded or only be read. If the driver 83 is open, the driver 80is blocked and vice versa.

The interval length is led from the RAM 79 via a data bus 86 and thedriver 83 to the input A of an adder 87. The input B of this adder 87receives a difference signal from the subtractor 88, which is formed asfollows. Digitized error signals are fed from a sensor measuring deviceaccording to FIG. 2 to a first storage element 89 via the data bus 76.In the case of each impulse transmitted by the entire unit, this storageelement 89 stores the signal currently present and leads the errorsignal stored in the preceding period to a second storage element 89,whose output leads to the input B of the subtractor 88. In this way, thesubtractor 88 determines in the case of each pulse the differencebetween two successive error signals, and supplies this to the input Bof the adder 87. The desired interval length is now increased ordecreased precisely by this difference if the difference is positive ornegative, respectively. The interval length corrected in this way istransferred to a period counter 90.

At the same time, a transfer is also performed to the low-phase counter91 by means of a signal which is shifted by hardware and thereby set athalf the value. The hardware shifting is performed by a data bus 92,which is reduced from 8 to 7 bits and in which the higher value linesare connected to the counter 91 by one bit lower in each case, and theleast significant bit is not connected at all. The two counters 90 and91 are now decremented by the incoming encoder pulses 93 and each givean output pulse to the flip flop 94 upon reaching the zero state. Thecounter 90 acts on the R input of the flip flop 94, as a result of whichthe output 95 thereof goes to the low level. The S input of the flipflop 94 is operated by the counter 91 approximately at half the period,as a result of which the output on the flip flop assumes the high level.

In this way, the output 95 conducts a pulse signal which is corrected bythe first actuating signal and in whose cycle it is now possible to readout a memory (not further represented) for the engraving pattern. Thememory module 96 and the counter 97 connected by a bus to it serve tocorrect the input zero signal 98 and effect the output of an output zeropulse 99 corrected by the first actuating signal. The RAM addresscounter 78 is controlled by the corrected output pulses of the output95. A digital changeover switch controlled by the bus control logic 77receives the first count signal via a RAM-write line 101 only uponstarting the unit, and all other counting pulses are the alreadycorrected output pulses of the circuit arrangement.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A method for producing a screen printing stencil comprising:exposing a thin-walled hollow cylinder having a light responsive layer on an outer surface thereof, said exposing comprising, while rotating said cylinder about its cylindrical axis, impinging a laser beam on said cylinder; focusing said laser beam onto said light responsive layer; moving said focusing along said cylindrical axis; switching said laser beam on and off in accordance with a desired stencil pattern; fixing at least one measuring position relative to said laser beam; determining, from said at least one measuring position, a radial deviation (Δs_(sens1) (k)) of the actual position of a wall of said cylinder from an ideal position for a multiplicity of circumferential positions of said cylinder; deriving a first actuating signal (Δt_(opt) <k+N.sub.Γ >) from said radial deviation obtained at said at least one measuring position; and displacing said stencil pattern in the circumferential direction of the cylinder in response to said first actuating signal, thereby compensating a tangential deviation of said wall from said ideal position.
 2. The method according to claim 1, further comprising:deriving a second actuating signal (Δs_(opt) <k+N.sub.Γ >) from said radial deviation; and adjusting said focusing of said laser beam in the radial direction after said cylinder has rotated over a circumferential section corresponding to an angular distance between said at least one measuring position and said laser beam thereby compensating for said radial deviation.
 3. The method according to claim 2, wherein said first actuating signal corresponds to said second actuating signal shifted in phase by a quarter revolution.
 4. The method according to claim 2, wherein deriving of said second actuating signal comprises determining said second actuating signal from at least three measuring positions situated at different circumferential positions.
 5. The method according to claim 4, wherein said at least three measuring positions are situated in only one cross-sectional plane of said cylinder.
 6. The method according to claim 1, wherein said deriving of said first actuating signal comprises deriving said first actuating signal from a multiplicity of radial deviations determined at a single measuring position.
 7. The method according to claim 1, wherein said deriving of said first actuating signal comprises determining said first actuating signal from a radial deviation determined from at least three measuring positions situated at different circumferential positions.
 8. The method according to claim 7, whereas said at least three measuring positions are situated in only one cross-sectional plane of said cylinder.
 9. The method according to claim 8, wherein said laser beam extends in said cross-sectional plane.
 10. The method according to claim 9, further comprising directing said laser beam radially onto said cylinder.
 11. The method according to claim 1, further comprising retrieving pattern information to be procured via said laser beam from a pattern information memory by means of said first actuating signal.
 12. The method according to claim 1, further comprising driving a deflecting device via said first actuating signal in order to deflect said laser beam in the circumferential direction of said cylinder.
 13. The method according to claim 1, further comprising generating overpressure in an interior of said cylinder. 